Once you have written a class, you can derive subclasses from it. This can save lots of painstaking code rewriting: you can take a class similar to the one you need to write, extend it to a subclass, and just modify the parts that are different. This is achieved using the extends operator.
The following example illustrates inheritance in PHP.
<?php
class Person
{
public $name;
public $age;
public function __construct($name, $age) {
$this->name = $name;
$this->age = $age;
}
public function get_age() {
return $this->age;
}
}
// Student is inherited from Person
class Student extends Person
{
public function sayHello() {
echo "Hello I am a student.";
}
}
class Employee extends Person
{
public function sayHi() {
echo "Hello I am an Employee.";
}
}
$std = new Student('Frank', 17);
$std->sayHello();
echo "<br>";
echo "I am " . $std->get_age() . "years old.";
echo "<br>";
$worker = new Employee('John', 35);
$worker->sayHi();
echo "<br>";
echo "I am " . $worker->get_age() . "years old.";
Inherited methods can be overridden by redefining the method in the child class. Note that the same method name should be used as in the example below:
<?php
class Person
{
public $name;
public $age;
public function __construct($name, $age) {
$this->name = $name;
$this->age = $age;
}
public function sayHello() {
echo "Hello I am " . $this->name . " and I am " . $this->age . " years old.";
}
}
// Student is inherited from Person
class Student extends Person
{
public $name;
public $age;
public $course;
public function __construct($name, $age, $course) {
$this->name = $name;
$this->age = $age;
$this->course = $course;
}
public function sayHello() {
echo "Hello I am " . $this->name . " and I am " . $this->age . " years old. I study " . $this->course;
}
}
$std = new Student('Frank', 17, 'Biology');
$std->sayHello();
To prevent overriding methods of the parent class, the final keyword is used. the following script will result to an error.
<?php
class Person
{
public $name;
public $age;
public function __construct($name, $age) {
$this->name = $name;
$this->age = $age;
}
final public function sayHello() {
echo "Hello I am " . $this->name . " and I am " . $this->age . " years old.";
}
}
// Student is inherited from Person
class Student extends Person
{
public $name;
public $age;
public $course;
public function __construct($name, $age, $course) {
$this->name = $name;
$this->age = $age;
$this->course = $course;
}
public function sayHello() {
echo "Hello I am " . $this->name . " and I am " . $this->age . " years old. I study " . $this->course;
}
}
$std = new Student('Frank', 17, 'Biology');
$std->sayHello();
Exercise
Create a class named Car with a property, name. Within the Car class, create a constructor setting the name. Also create a method, printDetails that prints the following: "Car Name: <name of car>". Create a child class (extending the parent) and name it Ford. Override the methods of the parent class to take another parameter, country, and the method to print: "Car Name: <name of car> - Country: <country>". Create an instance of the child class with name = "Ford" ; country = "USA".
<?php
<?php
class Car
{
public $name;
public function __construct($name) {
$this->name = $name;
}
public function printDetails() {
echo "Car Name: " . $this->name;
}
}
class Ford extends Car
{
public $name;
public $country;
public function __construct($name, $country) {
$this->name = $name;
$this->country = $country;
}
public function printDetails() {
echo "Car Name: " . $this->name . " - Country: " . $this->country;
}
}
$car = new Ford('Ford','USA');
$car->printDetails();
{
"test_output_contains":
{
"expected":"Car Name: Ford - Country: USA",
"error_message":"Sorry, the output is wrong."
},
"test_variable_exists":
[
{
"object":"$name",
"error_message":"Have you declared <code>$name<\/code>?"
},
{
"object":"$country",
"error_message":"Have you declared <code>$country<\/code>?"
}
],
"test_function_exists":
{
"object":"printDetails",
"error_message":"Did you create <code>printDetails<\/code>?"
},
"success_message":"Good job!",
"error_message":"There is something wrong on your code."
}